The top 8 mistakes in usability testing, on the Good Experience blog:
1. Not conducting any customer research and relying on your best internal guesses.
2. Conducting "pretend" research: "Let's pretend our user's name is Jane and you're 38..."
3. Conducting research, but the wrong type (focus groups instead of individual user tests)
4. Conducting one-on-one research, but with tasks defined beforehand.
5. Not inviting stakeholders to attend research (you know a better way to convince them of the severity of the results?)
6. Not prioritizing findings: "We uncovered 52 usability errors on the site, and here's a list of all of them."
7. Not relating to business objectives: the point of this work is to improve the business.
8. Missing the larger picture: understand the people, the human beings, who make the company possible.
[via Core77]